Which of the following best describes the mechanism(s) of action of CTLA-4?
**Core Concept**
CTLA-4 (Cytotoxic T-Lymphocyte Antigen 4) is a protein receptor that plays a crucial role in regulating the immune response by inhibiting T-cell activation. It is a key component of the immune checkpoint molecules that help maintain peripheral tolerance and prevent autoimmunity.
**Why the Correct Answer is Right**
CTLA-4 binds to its ligands, B7-1 (CD80) and B7-2 (CD86), on the surface of antigen-presenting cells (APCs). This interaction leads to the recruitment of tyrosine phosphatases, such as SHP-2 and SHP-1, which dephosphorylate and inactivate T-cell receptor-associated kinases (ZAP-70 and Lck). As a result, the downstream signaling pathways, including the PI3K/AKT and MAPK/ERK pathways, are inhibited, ultimately reducing T-cell activation, proliferation, and cytokine production.
**Why Each Wrong Option is Incorrect**
**Option A:** CTLA-4 is not an agonist that activates T-cell signaling. Instead, it acts as an antagonist to inhibit T-cell activation.
**Option B:** CTLA-4 does not function by blocking the CD28-B7 interaction. In fact, CTLA-4 competes with CD28 for binding to B7 molecules, but its binding leads to inhibition of T-cell activation.
**Option C:** CTLA-4 does not induce apoptosis in T cells. While it can inhibit T-cell activation and proliferation, it does not directly induce cell death.
**Clinical Pearl / High-Yield Fact**
CTLA-4 is a key target for cancer immunotherapy, and its blockade by monoclonal antibodies, such as ipilimumab, can enhance anti-tumor immune responses and improve patient outcomes.
**Correct Answer: C. CTLA-4 binds to B7-1 and B7-2 on antigen-presenting cells, leading to the inhibition of T-cell activation and proliferation.**